Conventionally, a filtering and condensing apparatus of a suction type has been adopted in order to condense agglomerated sludge generated at a water purification plan, for instance. One example of such an apparatus is disclosed by Japanese Patent Publication HEI03-23203.
This filtering and condensing apparatus comprises a sludge tank for containing sludge which is to be filtered and condensed, and a plurality of filtering plates disposed to be adjacent to each other so as to be aligned with each other in the sludge tank, each of which extends in a vertical direction of the sludge tank. Each of the filtering plates includes a supporting plate and a filtering cloth in a bag form which is integrally sewed on the supporting plate and houses the supporting plate to form a filtering chamber therein. A fixing frame is provided so as to surround each of the filtering plates. A filtrate discharging tube is communicated with the filtering chamber in order for the filtrate to be discharged through the filtering chamber. In addition, a suction portion for sucking the filtering cloth through the filtering chamber and an expansion portion for expanding the filtering cloth through the filtering chamber are provided, and coil springs for constantly exerting a tension force on each of the plurality of the filtering plates is disposed to be around the periphery of each of the filtering plate.
According to the above structure of the filtering and condensing apparatus, in case of filtering, a sludge in the sludge tank is sucked and thus filtered by the filtering cloths, so that the condensed sludge is deposited on the outer surfaces of the filtering cloths, while the filtrate which has passed through the filtering cloths and thus guided into the filtering chambers can be recovered outside of the sludge tank through the filtrate discharging tube.
On the other hand, in a case where the condensed sludge is recovered, the filtering cloths are expanded by supplying a compressed air to inner surfaces of the filtering cloths through the filtering chambers, whereby the condensed sludge deposited on the outer surfaces of the filtering cloths is separated, so that the condensed sludge deposited on the bottom of the sludge tank is discharged, and as a result, it can be incinerated or reclaimed as a cake by mechanically hydrating the discharged condensed sludge. It is possible to alleviate the load of the energy needed for the mechanical hydrating treatment such as pressurizing or depressurizing, etc. by said filtering and condensing of sludge.
In a case where the sludge is filtered or where the filtering cloth is expanded, since a tension force is constantly applied to each of the filtering plates by the coil springs provided on the periphery thereof, each of the filtering plates is maintained to be stable, even if each of the filtering plates flutters or swings to contact the adjacent filtering plate due to the suction force upon the filtering, or the air fed into the filtering chamber upon the expansion of the filtering cloth, whereby technical problems in which the filtering area is substantially reduced, or the condensed sludge deposited on the outer surface of the filtering cloth is separated can be prevented.
However, the following technical problems are caused in relation to a case where such a conventional filtering and condensing apparatus of suction type is made large-sized.
Firstly, in a case where the total filtering area of the filtering plates disposed within the sludge tank including a limited space is obtained when the filtering and condensing apparatus of the suction type is made large-sized, it is desired to increase the number of the filtering plates within the sludge tank as many as possible, while at the same time to maximize the filtering area of a single filtering plate. In order to do that, it is necessary to increase the filtering area by making the filtering cloth large, while to make the distance between the adjacent filtering plates as short as possible. However, even if the filtering plate can be maintained to be stable by the coil spring, the bigger the filtering cloth becomes, the larger the amount of the protrusion of the filtering plate toward the adjacent filtering plate becomes, whereby a risk that the filtering plate can contact the adjacent filtering plate is increased. On the other hand, in a case where the amount of the protrusion is limited, there occurs a risk that the filtering cloth is caused to be unduly tensioned due to the expansion of the filtering cloth, so that the normal filtering function cannot be attained, or the filtering cloth can be broken or torn, due to the fact that the fine holes of the filtering cloth are forced to be enlarged.
Secondly, in a case where the filtering cloth is integrally sewed on the corresponding supporting plate, when the filtering cloth is expanded in order to separate the condensed sludge therefrom, the supporting plate is forced to be contracted in the lateral direction (the direction substantially perpendicular to the vertical direction in which the sludge tank extends) upon the expansion of the filtering cloth, whereby the coil springs provided on both sides of the corresponding filtering plate are caused to be extended, so that the tension force by the coil springs is increased. If the coil springs are selected in accordance with such an increase of the tension force, the coil springs are caused to be contracted upon the filtering, on the contrary, whereby the tension force is caused to be decreased, so that there occurs a risk that the adjacent filtering plates can contact each other, while each of the filtering plate can flutter. The amount of such contraction of the supporting plate becomes inevitably large as the filtering plate becomes large.
Thirdly, in a case where the filtering cloth is immersed into the sludge for a long time, the filtering cloth made of a chemical fiber is caused to be swollen, whereby the corresponding supporting plate is caused to be extended, and as a result, the coil springs provided on both side of the filtering plate are caused to be contracted, causing the tension force by the coil springs to be decreased to generate the slackening of the filtering cloth. The amount of such an extension of the supporting plate becomes inevitably large, as the filtering plate becomes large.
Such being the case, even if the constant tension force is tried to be applied to the filtering plate by means of the coil springs based on the case of the filtering, the tension force is caused to be decreased due to the expansion of the filtering cloth, On the other hand, the tension force is caused to be increased due to the swelling of the filtering cloth, and such a fluctuation of the tension force becomes inevitably large, as the filtering and condensing apparatus of the suction type becomes large. The length of the coil spring may be lengthened in order to absorb such fluctuation of the tension force, however, the decrease of the filtering area in a limited space within the sludge tank is caused. In addition, the rigidity of the coil spring may be enhanced, however, the contraction of the supporting plate due to the expansion of the filtering cloth is restricted, whereby the filtering cloth is not fully expanded, so that the degree of the separation of the condensed sludge deposited on the filtering cloth is worsened, or the tension force is lost due to the fact that the coil spring remains fully extended because of the plastic deformation of the coil spring which is caused by the tension force being overcome by the expanding force of the filtering cloth.
In this respect, the coil springs used in the sludge for a long time may be preferably made of steel use stainless (SUS) which is an expensive and good corrosion resistant material, and may be standard products, not made-to-order products in a case where the filtering and condensing apparatus of the suction type is made large-sized, in view of the fact that tens of the coil springs are provide around the periphery of the filtering plate, and tens of the filtering plates are used.